Ubuntu installer with two disks: whihc one will be selected by "erase disk and install"

I have two disks on my computer, with:

• sda: backup disk (has 12.04)
• sdb: disk for system (has now 18.04)

Making a fresh install, I want to use the option erase disk and install Ubuntu (so that I can use encryption, otherwise I would just use option something else), on sdb, not sda. The installer correctly recognized there are two systems, but says: will delete all ... in all operating systems

How can I make sure it installs on sdb? Or will it really erase both disks?

Thanks!

After clicking Continue the next screen will ask you to confirm and shows which disks would be changed:

This is a very similar question to Will all my files be deleted if I choose 'Erase disk and Install Ubuntu' when installing in a VMWare virtual machine? although that one seems to be from the Ubuntu 18 era.

I just went through the process again on Ubuntu 20.04 with two drives: one with an old Ubuntu installation I wanted to replace and a second with an OEM Windows 10 installation. I selected the "Erase Everything and Install Ubuntu" rather than the "Erase Ubuntu 18.05.4 and Install Ubuntu" or the "Something else" options. Even though the button "Install Now" feels very final, clicking on it in this version actually takes you to a subsequent screen where you are asked to select which drive on which to "Erase Everything and Install Ubuntu".